1. Field of the Invention
This invention relates to a device and method for applying varnish to an electric wire, and more particularly to a device and method for forming an insulating layer with sure electric and heat insulation by applying a necessary minimum amount of varnish to the outer surface of an electric wire with no change in the mixing rate of resin and solvent in the varnish with good productivity.
2. Description of the Related Art
A previously known method for forming an electric insulating layer or a surface treated layer on the outer surface of an electric wire is to immerse the electric wire in an insulating liquid such as varnish by passing the electric wire through the tank containing the insulating liquid so that the varnish is applied to the outer surface using the viscosity of the varnish to form an insulating layer, and to dry the insulating layer of the varnish by passing the electric wire through a dry furnace so that the insulating layer is baked on the outer surface of the electric wire (see JP-A-9-237525 the contents of which are hereby incorporated by reference)
Another well known method for forming an insulating layer such as varnish on the outer surface of an electric wire is to roll a roller while an electric wire is being in movable contact with the roller, the roller being provided in a tank containing varnish, so that the varnish applied to the roller is applied to the outer surface of the electric wire.
However, in the previous method of forming the insulating layer disclosed in the above reference, the electric wire is immersed in an insulating liquid such as varnish by passing the electric wire through the tank containing the insulating liquid so that the varnish is applied to the outer surface using the viscosity of the varnish to form an insulating layer. Therefore, the varnish is exposed to air from when it is accommodated in the tank to when it is applied to the electric wire. As a result, the solvent mixed into the varnish is vaporized so that the mixing ratio of the resin component to the solvent component in the varnish varies momentarily.
Thus, it was difficult to maintain the viscosity of the varnish continuously to keep the applicability of the varnish to the electric wire effectively. In addition, as described above, the solvent is likely to be volatized so that it is difficult to maintain the quality of the varnish. The performance of insulation may be also lost and cracking is likely to occur, thereby reducing the production yield.
In order to obviate such inconvenience, by measuring the density of the varnish to the solvent and the viscosity of the varnish with passage of time, it was necessary to adjust the density or viscosity of the varnish if the solvent becomes insufficient. Such management or inspection of the density or viscosity of the varnish consumes much labor and time to form the insulating layer of varnish on the outer surface of the electric wire, thereby reducing the production efficiency.
In the above other method of applying the varnish onto the outer surface of the electric wire using the roller, the varnish is excessively stirred by the roller which is rolled in the tank. Further, by rolling of the roller, a certain amount of the varnish flies from the tank into the air, and the flown varnish sinks in the varnish in the tank again. Such repetitive behavior facilitates the vaporization of the varnish. Therefore, like the method of forming the insulating layer as disclosed in the above reference, the mixing rate of the resin component to the solvent component in the varnish is likely to vary. Accordingly, it is difficult to validate the applicability of the varnish to the electric wire by e.g. maintaining the viscosity of the varnish in an easily applicable state, and maintain the quality of the varnish. As a result, the varnish applied to the outer surface of the electric wire lacks an insulting performance, is apt to crack, which results in the product with low production yield.
Additionally, when the roller is rolling as described, air is mixed into the varnish to generate air bubbles. Therefore, pin holes were apt to occur in the insulating layer of the varnish applied to the outer surface of the electric wire. Further, the varnish is not applied to have a uniform thickness but applied unevenly. This generates cracks so that the insulating layer is apt to deteriorate, thereby loosing the electric insulation and heat insulation performance.